calculate and process downpayment

Hi everyone,
I use WC for a booking-site, or would like to do so. Our customers have to make a downpayment and pay the rest later (cash on location). It’s quite simple: whatever the cart-total is, upon checkout I would like to let my customer pay a certain percentage as downpayment.

[…]
cart-total: €1000
to be payed later: €800
to be payed now as a downpayment (20%): €200

That amount, €200, is what should appear in the checkout.

My question is: How would I go about to make this work. Can I simple use some hooks or do I have to make changes to the shopping-cart code?

Any help would be very much appreciated. Cheers!

http://ww.wp.xz.cn/extend/plugins/woocommerce/